Ab inito study of core-level shifts in nanoclusters and their correlation with the absorption energies on the surface.

XPS techniques are widely used for nanoclusters characterization. However, the comprehensive investigation of the spectral features and their origin has not been performed. In this work, we investigate the shifts of the core-level binding energies in small gold nanoclusters by using ab initio density functional theory calculations. The shift of the 4f states is calculated for magic number nanoclusters in a wide range of sizes and morphologies. We find a non-monotonous behaviour of the core-level shifts in nanoclusters depending on the size. We demonstrate that there are three main contributions to the Au 4f shifts, which depend sensitively on the interatomic distances, coordination and quantum confinement. They are identified and explained by the change of the on-site electrostatic potential.
In order to investigate the correlation between the shifts of the core states and catalytic activity, we have studied Pd nanocluster with icosahedral and octahedral morphology. The absorption of ethylene was modelled for different sites and it was shown that it is well correlated with the core-level shifts. Thus, we show that XPS spectra may provide means for characterization of nanoclusters catalytic properties.